This proposal is concerned with analysis of the recurrent Renshaw circuit with the goal of contributing to our understanding of the role of recurrent inhibition in controlling the discharge of spinal motoneurons. The analysis of this disynaptic circuit will be accomplished primarily by the use of a technique for stimulating single motor axons in conjunction with spike-triggered averaging in order to record the small postsynaptic potentials in spinal motoneurons which result from such stimulation. Using these methods a number of specific issues will be addressed. The characteristics of single-axon recurrent inhibitory postsynaptic potentials (RIPSPs) will be documented as well as the relation of these characteristics to interneuronal discharge. The distribution of RIPSPs within a homonymous motor nucleus will be determined with respect to the motor-unit type of the stimulated axon and the motor-unit type of the motoneuron receiving the recurrent inhibition. The distribution of RIPSPs will also be examined for evidence of a "partitiioning" of RIPSPs, i.e., that RIPSPs produced by asons innervating a particular region of muscle are largest in those motoneurons which innervate that same region of muscle. Such a partitioning, if found, will be analyzed for contributions of topographic specificity in comparison to species specificity. These studies are expected to contribute to our understanding of the organization and functional significance of recurrent inhibition to spinal motoneurons and, in addition, to provide an approach for analyzing neuronal circuits in the mammalian spinal cord which involve di- and polysynaptic connections. The ultimate health-related goal of this study is to contribute to an understanding of altered function of segmental motor control mechanisms, such as occurs in such states as spasticity, rigidity and remor.